Italy's top criminal court has scathingly faulted prosecutors for presenting a flawed and hastily constructed case against Amanda Knox and her former Italian boyfriend

The Court of Cassation said it threw out their convictions for the 2007 murder of her British room- mate in part because there was no proof they were at the crime scene.

The court issued its formal written explanation, as required by Italian law, for its March ruling vindicating the pair once and for all in the murder of Meredith Kercher, in the apartment the two women shared while students in Perugia, Italy.

The court wrote there was an "absolute lack of biological traces" of Ms Knox, an American, or co-defendant Raffaele Sollecito in the room or on the victim's body.

Criticising the quality of the prosecution's case, the court cited "blameworthy omissions of investigative activity".

Media clamor also was a factor in what was ultimately a flawed case, the high court concluded.

"The international spotlight on the case in fact resulted in the investigation undergoing a sudden acceleration, that, in the frantic search for one or more guilty parties to consign to international public opinion, certainly didn't help the search for substantial truth," the judges wrote.

The high court in March declared that Ms Knox, now 28, and Mr Sollecito, now 31, did not murder the 21-year-old, a stronger exoneration than merely finding there was insufficient evidence to convict.

Had the Cassation Court upheld the 2014 appeals court convictions of the pair, Ms Knox would have faced 28-and-a-half years in an Italian prison, assuming she would have been extradited from the United States, while Mr Sollecito had faced 25 years.

The pair had served nearly four years in Italian prison after a first, lower court conviction by a Perugia court.

They had always proclaimed their innocence. A man from Ivory Coast, Rudy Hermann Guede, was convicted in separate proceedings and is serving a 16-year sentence.

The Cassation Court's written explanation is tantamount to a "great censure, a note of solemn censure of all the investigators," Knox defence lawyer, Carlo Dalla Vedova, said.

The Cassation panel of five judges essentially concluded that while there were indications Guede could have had accomplices, nothing in the prosecutors' case proved that either Ms Knox or Mr Sollecito were involved in the murder.

It also wrote that the lower court ignored expert testimony that "clearly demonstrated possible contamination" of evidence and misinterpreted findings about the knife allegedly used to slit Ms Kercher's throat, in what prosecutors had described as a sexual assault.

On more than one of the 52 pages of explanation, the judges agreed with the defendants' contentions that the convictions were a "violation of the principle of a reasonable doubt".